1. Field of the Invention
The present invention generally relates to a decoder, in particular, to a decoder of a digital-to-analog converter.
2. Description of Related Art
Binary digital impulses appear as long strings of ones and zeros, and have no apparent meaning to a human observer, but when a digital-to-analog converter (DAC) is used to decode the binary digital signals, meaningful output appears. The output might be a voice, a picture, a musical tune, or mechanical motion. The digital-to-analog converters transform a digital word representing an analog value such as a voltage into an output corresponding to that analog value.
FIG. 1 shows a function block of a digital-to-analog converter. Output Outs is selected from gamma voltage Vr0˜Vr2n−1 by corresponding decoder input which is controlled by digital codes A0˜An−1 and A0˜ An−1. Here, we choose An−1 as Most Significant Bit (MSB) and A0 as Least Significant Bit (LSB). The output voltage Outi passes through OP buffer to bump pad Pouti.
Conventionally, the switch array (i.e. decoder) of the digital-to-analog converter (DAC) in the line-inversion LCD source driver is consisted of P-type Metal-Oxide-Semiconductor (PMOS) and N-type Metal-Oxide-Semiconductor (NMOS) transistors, which is controlled by digital codes A0˜An−1, and A0˜ An−1, as shown in FIG. 2. Each gamma voltage Vr0˜Vr2n−1 is transmitted through transmission gate logic consisting of PMOS and NMOS transistors. When the switch arrays are disposed on same horizontal for each output voltage, the pitch of each channel for the output voltage should be wide enough. Unfortunately, source driver has limited width, and foundry also has mask reticle restriction. When the switch arrays are not disposed on same horizontal, twice routing-traces for the gamma voltage in the source driver would be required and the area of the source driver is increased.
FIG. 3 is a partial layout of the NMOS and PMOS switch arrays on the same horizontal, which uses conventional contact device. Referring to FIG. 3, when the contacts are disposed in the same row of the switch array, only a few contacts are needed to dispose between the gamma voltages, such that the bump pad pitch would not be increased. However, when the contacts are disposed in the same column of the switch array, the contacts are close to each other and more space between the rows of the switch array would be required for mask reticle, such that the bump pad pitch of the switch array is increased.